Earth Planets Space, Vol. 50 (Nos. 6, 7), pp. 587-593, 1998
Information Processing Center, Kobe University, Rokko-dai 1-1, Nada, Kobe, Japan
(Received December 8, 1997; Revised January 7, 1998; Accepted January 25, 1998)
Abstract: Dust particles in the disk of Beta Pictoris (bPic) is modeled by fluffy aggregates consisting of slightly modified interstellar grains. We calculate the optical properties by Discrete Dipole Approximation with a1-term method. It is found that the characteristic size of an aggregate is defined by the radius of volume-equivalent sphere. The wavelength dependence of the scattering efficiency in visible becomes flat when the volume-equivalent sphere exceeds 1 mm irrespective of the monomer size, while the infrared silicate feature is present even for aggregates larger than 10 mm. Therefore, our model can account for the coexistence of the neutral scattering and silicate feature in the bPic disk without detailed tuning of the dust size distribution. Due to the enhancement of geometrical cross section, aggregates generally show higher scattering and absorption efficiencies in visible compared with the volume-equivalent sphere. In contrast, the absorption efficiency for an aggregate is comparable to that for the volume-equivalent sphere when the size is smaller than wavelength. These properties are also consistent with the observed superheat and high albedo of the dust particles in the bPic disk.